A hydraulic lash adjuster, of the type capable of forming a pivot for a cam actuated finger follower in an engine valve train, commonly consists of two major elements, a body and a plunger. The body is a cup-shaped or cylindrical member having a peripheral outer wall with closed and open ends, and is mountable directly in a cylinder head or other camshaft mounting member of an engine. The plunger has the general form of a hollow piston with a rounded top, which acts as a pivot for an associated finger follower.
The plunger is reciprocably received within the body with close clearance for controlling the leakage of oil between the adjoining surfaces. An inner end of the plunger includes means defining a wall with a check valve controlled orifice leading from an oil reservoir within the plunger to an enclosed space between the lower plunger wall and the closed end of the body which forms a high pressure chamber. The oil reservoir is supplied from the engine through openings in the walls of the body and plunger, and an opening in the rounded outer end supplies oil to lubricate the finger follower and associated components.
In operation, when the associated engine valve is closed and the finger follower engages the cam on its base circle, a plunger spring in the high pressure chamber forces the plunger outward to take up lash between the plunger and finger follower, and thus remove all lash from the valve train. This lowers the pressure in the high pressure chamber so that oil is drawn from the reservoir in the plunger through the check valved orifice into the high pressure chamber which is maintained full of oil. During the next valve opening cycle, the reaction force from the engine valve spring acts downwardly against the plunger, increasing pressure in the high pressure chamber and forcing some of the oil therein out of the high pressure chamber through clearances between the plunger and body. During operation at normal engine temperatures, this oil is replaced by makeup oil from the plunger reservoir when the valve is closed on the next phase of its operating cycle.
During start up of a cold engine, oil viscosity is high and exhaust valve growth is rapid so that hydraulic lash adjusters which use a spring biased plunger may not provide a sufficient leakdown rate to avoid holding the valve off its seat on the cam base circle, a condition sometimes called thermal pump up. This condition may cause improper engine operation or stalling and thus requires correction.
Mechanically lashed valve trains provide sufficient lash to accommodate transient growth of valve train components following start up. However, they do not have the capability of automatically compensating for build tolerances and wear over the life of the engine as hydraulic lifters do. Means for correcting the thermal pump up problem while retaining the benefits of hydraulic lash adjusters are accordingly desired.